With the development of more and more sophisticated electronic devices, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components like laptop computers, cell phones, personal digital assistants (PDAs), digital cameras and the like; high capacity and response memory components such as hard drives; electromagnetic sources such as light bulbs in digital projectors, as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. Indeed, electronics located in the dashboard of vehicles, and the like, can also be the source of high temperatures.
With the increase in heat generation in such devices over the recent past, hot spots, that is, localized high temperature areas, have become an issue. For instance, the increasing desire for smaller and more compact electronics, such as laptop computers, cell phones, PDAs, digital cameras and projectors, etc., means that the heat source (such as a hard drive, central processing unit (CPU), graphics chip, inverter/converter, memory chips, light sources, and the like) will be adjacent external surfaces of the device (even the dashboard of a vehicle). As such, external surfaces of the device can become heated, which can be uncomfortable or even dangerous to the user.
For instance, in some laptop computers, the hard drive, which can generate substantial amounts of heat, is located beneath one of the so-called “palm rests” of the computer—the area between the keyboard and the user, where the users palm is normally resting while typing. In thinner laptops, the heat generated by the hard drive can be transferred through the laptop case to the user's palm, causing discomfort and even pain. Indeed, in some laptops, the palm rest temperature has been measured to be higher than 40° C. Likewise, heat-generating components can cause the bottom of a laptop computer to become heated, causing discomfort or even pain to the user when the laptop is positioned in the user's lap. This has become a significant problem for makers of laptop computers, and other portable devices where there is a continuing effort to make the devices smaller for greater portability.
One potential solution is to insulate the heat source; however, this is not a satisfactory solution, since doing so keeps the heat generated by the heat source concentrated at the heat source, potentially damaging the heat source. Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an increase in the processing speed, reliability and life expectancy of the device. Thus, concentrating heat generated at the heat source can be counterproductive.
One group of relatively light weight materials suitable for use in the dissipation of heat from heat sources such as electronic components are those materials generally known as graphites, but in particular graphites such as those based on natural graphites and flexible graphite as described below. These materials are anisotropic and allow thermal dissipation devices to be designed to preferentially transfer heat in selected directions. The graphite materials are much lighter in weight and thus provide many advantages over copper or aluminum.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.0 g/cm3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
While the use of sheets of compressed particles of exfoliated graphite (i.e., flexible graphite) has been suggested as thermal spreaders, thermal interfaces and as component parts of heat sinks for dissipating the heat generated by a heat source (see, for instance, U.S. Pat. Nos. 6,245,400; 6,482,520; 6,503,626; and 6,538,892), the issues of “touch temperature,” that is the heating of external surfaces of a device to the extent that the user is discomfited or endangered has not adequately been addressed.
Accordingly, there is a continuing need for improved designs for thermal solutions for equipment containing heat-generating components, including portable or miniaturized devices, where the heat from the component needs to be dissipated while avoiding discomfiting or endangering users of the device.